Allow for naming the rootId with no options

Right now if you want to name your rootId something different, but don't pass
in any options, it doesn't change the name of your rootId (see confusion in
github #49.

This would allow someone to use

```js
const app = choo()
app.start('myRootNode')
```

Rather than having to use

```js
const app = choo()
app.start('myRootNode', {name: 'myRootNode'})
```

index.js

@@ -54,7 +54,7 @@ function choo () {
   // start the application
   // (str?, obj?) -> DOMNode
   function start (rootId, opts) {
-    if (!opts) {
+    if (!opts && typeof rootId !== 'string') {
       opts = rootId
       rootId = null
     }